until the angels are fully grown, or even mostly grown, I'd call that medium
stocking. The kind of otos generally in the trade do not get large, although
they can get very fat - they'll max out about 2-3". Cardinal tetras are a
natural food source for angels in the wild, but until your angels get big,
it's not a problem, and may not be even then.

At our LFS, however, they did have an angelfish try to eat an oto, and the
oto's barbs got stuck in the angel's throats, and it swam around with this
oto hanging out of its mouth. I don't know the end of the story, I keep
forgetting to ask.